1. Field of the Invention
The invention concerns a method to generate magnetic resonance (MR) slice exposures of an examination subject, as well as an MR measurement data processing unit to generate MR slice exposures, and an MR system that, among other things, has such an MR measurement data processing unit.
2. Description of the Prior Art
In order to obtain magnetic resonance slice exposures (i.e. image data generated with a magnetic resonance tomograph apparatus) from a region of the inside of the body of an examination subject, the body or the body part to be examined must initially be exposed to an optimally homogeneous, static basic magnetic field, which is most often designated as a B0 field. The nuclear spins in the body are thereby aligned parallel to the direction of the B0 field (typically designated as the z-direction of a Cartesian coordinate system). Moreover, radio-frequency pulses (RF pulses) having a frequency in the range of the resonance frequency (known as the Larmor frequency) of the nuclei to be excited in the present B0 field, are radiated into the examination subject with suitable radio-frequency antennas. The spins of the nuclei to be excited—normally hydrogen nuclei since water is abundantly present in the body—in the examination subject are excited by these radio-frequency pulses such that they are deflected out of their rest state, parallel to the basic magnetic field B0, by an amount known as the “excitation flip angle.” The excited nuclear spins then initially precess around the z-direction and relax again bit by bit, the relaxation being dependent on the chemical shift and the molecular environment in which the excited nucleus is located. The magnetic resonance signals generated upon relaxation are acquired as raw data by radio-frequency reception antennas, and the magnetic resonance images are ultimately reconstructed on the basis of the acquired raw data. Spatial coding takes place with the use of rapidly switched gradient magnetic fields that are superimposed on the basic magnetic field B0 during the emission of the magnetic resonance radio-frequency pulses and/or the acquisition of the raw data.
Typical magnetic resonance imaging is based on the sequential excitation and readout of arbitrarily oriented slices in the examination subject. In order to cover a three-dimensional volume of a specific volume known as a “region of interest” (ROI), measurement data (raw data) are typically generated by means of a succession of slice measurement sequences in order to obtain a complete stack of parallel measurement slices through the examination subject that are spatially shifted relative to one another in a defined stack direction (for example in the z-direction). This slice stack can be constructed so that one slice immediately adjoins the next slice, such that the volume is seamlessly covered. However, it is also possible to acquire the slices with a determined slice interval. The defined excitation of a slice can take place by radiating a radio-frequency pulse with simultaneous application of a slice selection gradient in the stack direction, for example in the z-direction. The thickness of the selected slice is determined by the bandwidth of the RF pulse as well as the amplitude of the slice selection gradient; the slice position is determined by the Larmor frequency present at the respective location, which Larmor frequency is dependent on the magnetic field B0 that is present at the respective location.
In this typically used type of slice excitation, the problem exists that the excited nuclei in the body tissue have no uniform precession frequency in the magnetic field; rather, they can differ according to their chemical environment for different tissue types. This is typically designated as a chemical shift. In magnetic resonance imaging, the chemical shift of fat tissue in relation to the typically excited hydrogen nuclei of water is particularly interfering, since fat occurs in significant quantities in many body regions. The chemical shift between fat tissue and water amounts to approximately 3.5 ppm. Upon slice excitation, the effect of the chemical shift has the effect that the signal of tissue with deviating frequency is shifted in the stack direction. In the extreme case, this can lead to the situation that the fat signal in the image data generated from the raw data originates from a completely different slice position, and thus contrasts and anatomy are adulterated in the diagnostic image generated later from this image data.
This problem intensifies with increasing field strength of the B0 field, for two reasons. The slice offset increases proportionally with the field strength. For example, given a slice thickness of 3 mm and a bandwidth of the exciting RF pulse of 1 kHz, at a field strength of 1.5 Tesla the fat signal is shifted by approximately 0.63 mm (which constitutes approximately 23% of the slice thickness) relative to the water signal. At 3 Tesla, this shift doubles relative to a 1.5 Tesla magnetic field, meaning that it then amounts to 1.26 mm (which makes up 42% of the slice thickness). At 7 Tesla, the distance already amounts to 2.98 mm (which constitutes 98% of the slice thickness). While the shift thus appears to still be acceptable give routine examinations at 1.5 Tesla systems, the fat signal at 7 Tesla systems already originates from a completely different slice position, which can lead to significant problems in the later diagnostic evaluation.
An additional reason why the problem intensifies with increasing magnetic field strength is that, at higher field strengths, radio-frequency pulses with lower bandwidth should be used in order to reduce the radio-frequency exposure of the patient, known as the SAR (specific absorption rate). Although the use of RF pulses with lower bandwidth is counterproductive for image artifacts that occur due to the chemical shift, this is nevertheless frequently done in order to avoid otherwise necessary SAR limitations.